Use -Werror in system/core

* Move -Wall -Werror from cppflags to cflags.
* Fix/suppress warning on unused variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I1e05e96a1d0bcb2ccef1ce456504b3af57167cc5
diff --git a/trusty/keymaster/Android.bp b/trusty/keymaster/Android.bp
index 6b9d723..7735684 100644
--- a/trusty/keymaster/Android.bp
+++ b/trusty/keymaster/Android.bp
@@ -30,6 +30,7 @@
         "trusty_keymaster_ipc.cpp",
         "trusty_keymaster_main.cpp",
     ],
+    cflags: ["-Wall", "-Werror"],
     shared_libs: [
         "libcrypto",
         "libcutils",